Derivatives of polyethers and of pentacyclic heterocycles, their polymers and their applications, particularly to the complexing of metal ions

ABSTRACT

It concerns a monomer, a polymer obtained from the monomer and a process for obtaining the said polymer. The monomer consists of a polyether containing at least 3 ether units and pyrrole or thiophene units bonded to the ends of the chains of this polyether via, for each of these heterocycic compounds, either their carbon atoms in the 3,3&#39;-positions of their heterocycles or, as regards more particularly the pyrrole groups, via their respective nitrogen atoms, the 2- and 2&#39;-positions of these heterocycles being, however, free of all substitutions or, at the very most, substituted by easily removable groups (protective groups, for example). 
     Application to the purification and recovery of metal ions such as radioactive silver.

Salts of polymers of compounds of aromatic nature possessing complexingcavities have already been produced by molecular electrochemistrytechniques (1), these techniques involving an electrodeposition ofcertain polytriphenylenes on essentially non-corrodible supports, suchas precious metals (platinum, gold) or vitreous carbons. The most commonexample relates to the polymerization of the compound known under thename DB-18-C-6 (I) to polytriphenylene: ##STR1##

The direct formation of a doped form of the polymer (doubtless of redoxnature) has been demonstrated. These polymer salts can be easily reducedor "dedoped" by chemical agents such as tertiary amines ortetraalkylammonium hydroxides. It is possible, in this way, to obtainneutral, non-conductive resins which are resistant to chemical agentsand to solvents, which have an electronic conductivity and which haveboth poly-aromatic linkages and polyether units, obtained from certainbiaromatic polyethers (II), ##STR2## in which the anodic activation isincreased by the presence of donor groups on the aromatic systems(example: ether groups). When the polymerization is carried out at asufficiently oxidizing potential, conductive polyparaphenylenes can beobtained by virtue of the high concentration of redox sites produced inthe matrix (oxidation at 4 electrons per monomer unit). These redoxsites make the material sufficiently permeable to the flows of electronsnecessary for the growth in thickness of the films formed, by virtue ofthe electron jumps induced between these sites, during the developmentof the deposits. It has already been proposed to use such polymers forextracting certain metal ions present in aqueous or organic solutionsbrought into contact with them (2). However, they are not alwayseffective, as regards, for example, removing (or recovering) traces ofsilver contained in certain effluents. The production costs are high.Difficulties also accompany the synthesis of the monomers, especiallyfrom hydroquinones, and then the oxidation of these monomers, which mustbe carried out at a fairly strongly oxidizing potential even if the arylgroups have been activated beforehand.

The aim of the invention is the production of new monomers which aremore easily accessible than the aromatic polyethers of the typementioned above and which are polymerizable by the use of molecularelectrochemistry techniques of the abovesaid type but at less oxidizingpotentials. Consequently, another aim of the invention is the easyextension of these techniques to other categories of conductive supportsmore sensitive to oxidation than the precious metals indicated above. Inparticular, the aim of the invention is the formation, more easily thanpreviously, of deposits of this type on graphite supports. The use ofthe latter as a support for aromatic polyethers of type II wasdifficult, due to the ability of the graphite to be at least partiallyconverted to graphite salts at relatively low oxidation potentials,sometimes below the potentials required for obtaining anelectrodeposition of the corresponding polymers.

A further aim of the invention is the production of polyetherderivatives which are particularly efficient as regards their ability toremove traces of certain metals, in concentrations below 10⁻¹⁰ M, indeedeven 10⁻¹² M. In particular, the metals in question are those whosesequestration or encapsulation is being sought: particularly thetransition metals, zinc, cadmium, silver and the like or radioactivemetal elements.

The monomers according to the invention, which are also polymerizable bymolecular electrochemistry techniques, consist of polyethers containingat least three ether units and carrying pyrrole or thiophene unitsbonded to the ends of the chains of these polyethers via, for each ofthese heterocyclic compounds, either their carbon atoms in the3,3'positions of their heterocycles or, as regards more particularly thepyrrole groups, via their respective nitrogen atoms, the 2- and2'-positions of these heterocycles being, however, free of allsubstitutions or, at the very most, substituted by easily removablegroups (protective groups, for example).

The polyether chains which form part of the constitution of the monomersaccording to the invention are preferably linear and contain etherunits, preferably of the type --[(CH₂)_(p) --O--]_(n) -- with p being aninteger of 2 or 3 and n being an integer from 3 to 5, these chains thenbeing connected at the α- and ω-positions to pyrrole or thiophene unitsunder the conditions indicated above.

In particular, these chains are of the type:

    --(CH.sub.2).sub.q' --[(CH.sub.2).sub.p' --O--(CH.sub.2).sub.p" ].sub.m --(CH.sub.2).sub.q" --

with

q' and q" being, independently of one another, equal to 1 or 2,

p and p" being, independently of one another, equal to 1 or 2.

The invention more particularly relates to new monomers of formula:##STR3##

Such monomers are capable of being obtained easily by reaction of analkali metal salt of pyrrole, particularly pyrrolylpotassium, with adihalogenated derivative of the glycol corresponding to the finalpolyether chain. For example, the corresponding glycol will have beenconverted beforehand to the dichlorinated derivative by reaction withthionyl chloride.

Other categories of preferred monomers in accordance with the inventionare characterized by one or other of the following structures: ##STR4##and more particularly those corresponding to the following formula:##STR5## with X being an NH group or a nitrogen or sulphur atom, q' andq" being, independently of one another, equal to 1 or 2 and m beingequal to an integer from 3 to 5.

The advantage of such molecules in the anodic polymerization process isto give rise to very much larger heterogeneous polymerisation yields[(weight of electrodeposited resin/monomer mass ratio)×100]. It isadvisable to leave the 2,2'-positions on the pyrrole and thiophene ringsfree so as not to slow down the polymerization process.

Thus, with the thiophenes and when p=0 and p'=0, the coupling of apolyether chain can be carried out without problems from3-bromothiophene in accordance with the reaction model developed below.##STR6## This reaction is carried out at ordinary temperature in etheror THF for 24 hours. X in the starting halide RX=Cl or Br. L is a ligandof the phosphine type (Ref.: M. KUMADA et al., J. Org. Chem., (1981),46, 4481 and J. P. MONTHEARD et al., Synth. Con., (1984), 14, 289). Inthis case, the organo magnesium derivative formed will be a"diorganomagnesium" derivative of the type R' (MgX)₂ which can be easilyprepared from α,ω-diols containing a polyether chain and dichlorinatedwith thionyl chloride. ##STR7##

The substitution reaction in the 3-position of the pyrroles (inparticular in the case where p=0 and p'=0) is, in principle, moreproblematic because it assumes a priori the protection of the N--H--functional group. A possible procedure is the protection by a sulphonylchloride (R=methyl or phenyl): ##STR8## and then carrying out aFriedel-Crafts reaction with acetyl chloride in the presence ofaluminium chloride. The ketone is then reduced to the correspondingalcohol by lithium aluminium hydrate with a good yield. ##STR9##

The etherification reaction is carried out in THF in the presence ofsodium hydride while the ditosylate of the polyether diol isprogressively added. Deprotection of the amino functional group of thepyrrole is then carried out conventionally by phase transfer catalysis(presence of a base in two-phase medium in the presence of a detergent,such as that marketed under the trade name Triton B).

The introduction of acid groups (carboxy and sulpho) in the 3'-positionon the pyrrole and the thiophene makes it possible to greatly reinforcethe ion-exchange resin nature of this type of polymer. ##STR10##

However, the difficulty relating to the synthesis of such monomers apriori excludes their use. It is preferred to carry out acopolymerization of: ##STR11## with the polyether monomers 1 or 2 inequimolecular mixtures (there is thus statistical introduction of 0.5sulpho group per aromatic ring).

In the case of the compounds of formula VI in which p and p' are otherthan 0, the initial corresponding thiophene or pyrrole compounds willadvantageously have OH groups in the 2- and 2'-positions respectivelywhich are then capable of taking part in a reaction with a polyetherditosylate corresponding to the polyether chain which must bind theseheterocycles together.

The invention also relates to the oligomers (including dimers) and thepolymers capable of being produced from the abovesaid monomers, thesepolymers being characterized by polymeric chains containing a pluralityof pyrrole or thiophene rings, if appropriate both at the same time, andinterconnected to each other via 2,2'-bonds, on the one hand, andpolyether chains, corresponding in particular to the formulae --CH₂--(--CH₂ --O--CH₂ --)_(m) --CH₂ -- with m equal to 3, 4 or 5, thesechains being themselves bonded by their own ends either to the carbonatoms in the 3,3'-positions respectively of these pyrrole or thiophenerings or, in the case of the pyrrole rings, to their nitrogen atoms.

A process for the production of these polymers comprises theelectrodeposition of the polymer on an anode of an electrolysis cell, oron a conductive support material, particularly carbon-based, inelectrical contact with this anode, from an electrolyte containing themonomer as defined above and from a salt capable of giving rise to anon-oxidizable anion, under a potential which allows a polymerization ofthe monomer on contact with the anode or, when it is present, with thesupport material, the abovesaid monomers and salts being in thedissolved state in a polar or dipolar organic solvent, this solvent andthe support being respectively chosen from those which themselves have asufficient resistance to oxidation under the abovesaid potential.

It goes without saying that this process must be implemented in theabsence of any trace of the specific metal or metals, particularlyalkali metals or alkaline-earth metals, transition metals, zinc,cadmium, silver and the like or radioactive elements, when the polymersproduced are intended to be used for the sequestration or encapsulationof these metals, particularly in the form of metal ions, moreparticularly in the trace condition, in liquid media brought intocontact with them for purposes either of purification or of recovery.

The polymerization of the dipyrrole or dithiophene monomers can becarried out on any non-corrodible surface such as platinum, gold orrhodium. Depositition is also possible on all vitreous carbons andvarious types of graphite [natural or "HOPG" (abbreviation of theexpression "Highly oriented pyrolytic graphite")]. It can be easilycarried out in the usual polar and dipolar organic solvents, if theyhave a reduced electrochemical activity (by significant oxidation of thesolvent before +1.2 V/Ag/Ag⁺). Thus, technical grade acetonitrile caneasily be used but dichloromethane and nitromethane can also lead to thepolymerization of the pyrroles. The electrolyte must essentially containa non-oxidizable anion. Thus, tetraalkylamonium (alkyl=Me, Et, Pr, Bu,Hex, d and the like) and alkali metal (lithium, sodium or potassium)perchlorates, hexafluorophosphates, tetrafluoroborates,hexafluoracetates and the like are all electrolytes which can be used.The choice of the salt can be guided by the solubility in the solventunder consideration. The polymers are deposited particularly well. Theycan be obtained on the various forms of carbon, in particular graphite,and can be obtained in the form of much thicker deposits than onprecious metals.

The invention also obviously relates to the polymers supported by theseconductive supports, particularly based on carbon and more particularlyon graphite. The use of carbon, in particular graphite, supports is ofparticular advantage, taking into account the ability of these materialsto be brought under various forms, particularly as porous grains whichcan then be charged with polymers.

The invention also naturally relates to the applications which can bemade of these polymers in the supported or non-supported state. Inparticular, the invention relates to the application of these polymersas sequestration or "encapsulation" agents for metal ions, moreparticularly resulting from transition metals, zinc, cadmium, silver andthe like, contained in solutions brought into contact with them, forpurposes either of purification of these solutions or else of recoveryof the corresponding metals. The polymers of the type in question maythen be regarded as "polymer-cages" with respect to these ions.

The low molecular weight oligomers, and more particularly the dimers,particularly cyclic dimers which are fairly soluble in organic solvents,can, for this reason, be the object of various industrial applications:phase transfer catalysis, application to the separation of rare-earthmetals, and the like.

Other characteristics of the invention will become further apparent inthe course of the description which follows of certain examples whichhave no other objective than that of illustrating the invention withoutlimiting it in any way. Reference will also be made to the drawings inwhich:

FIG. 1 shows nuclear magnetic resonance spectra relating to a specificmonomer in accordance with the invention ¹³ C NMR spectra, 75 MHz,CDCl₃, δ(ppm/TMS);

FIG. 2 is a diagram of an electrolytic cell capable of being used forthe production of a polymer in accordance with the invention,particularly in the supported state on a conductive support;

FIG. 3 is a diagram illustrating the monitoring of the formation of thepolymer layer in the electrolysis cell in the course of operation;

FIGS. 4 and 5 are reproductions of photographs showing the morphology ofpolymer deposits obtained on graphite grains under the conditionsdescribed in the example, after dedoping with Bu₃ N contained in CH₃ CN,at different magnifications, respectively 18,700 and 16,000 times. Thesamples were gilded with fine gold;

FIG. 6 shows a diagram of an embodiment of a filter constituted byimplementing the process for the production of a polymer according tothe invention.

EXAMPLES A) Preparation of the monomers (III)

The method of synthesis applicable to all the polyethers (1≦m≦5) isdescribed below.

1. Typical synthesis example for the triether (m=3):1-11 bis(1--1pyrrole) 3,6,9 trioxaundecane

It is advisable first of all to synthesize potassium pyrrole: ##STR12##40 ml of THF, treated beforehand with the benzophenone-sodium system inorder to remove peroxides, are placed in a 250 ml, 3-necked,round-bottomed flask. 8 g of potassium (2.05·10⁻¹ mol) are added insmall portions. 14.42 g of pyrrole (2.15·10⁻¹ mol) are then addeddropwise via a dropping funnel. The mixture is left stirring (at roomtemperature) in an inert atmosphere (argon) for 24 hours. The contentsare filtered, washed with ether and then dried. 16.5 g of dry salt areobtained in this way (yield approximately 75%).

The bielectrophile is obtained at the same time by treatment oftetraethylene glycol with SOCl₂ according to the procedure described byC. J. Pedersen (J. Am. Chem. Soc. (1967), 89, 7017): ##STR13##

The reaction is carried out in a 500 ml round-bottomed flask (withreflux condenser, nitrogen inlet, thermometer and dropping funnel) inwhich 43 g of tetraethylene glycol (0.22 mol), 200 ml of toluene and38.6 g of pyridine (0.49 mol) are placed. The mixture is brought, withstirring, to 85° C. and 58 g of thionyl chloride (0.49 mol) are addedvery slowly. This requires approximately 4 hours; the temperature of thereaction mixture is subsequently maintained at 80° C. for 20 hours. AnHCl solution (5 ml of concentrated acid in 20 ml of water) is thenadded. Separation is carried out by settling, washing is carried outwith water and drying is carried out over CaCl₂. 46 g of crude liquidare obtained in this way. 40 g of pure dichlorinated product areisolated by vacuum distillation (yield 80%). Condensation of the pyrrolesalt with the bielectrophile is then carried out in order to yield themonomer. ##STR14##

The condensation is carried out in a 250 ml, 3-necked, round-bottomedflask in which there are placed:

25 ml of dimethyl sulphoxide (DMSO)

15.75 g K pyrrole (1.5·10⁻¹ mol), thus in 50% excess

50 ml of treated and distilled THF.

The chlorinated bielectrophile (11.55 g, 0.5·10⁻¹ mol) in solution in 40ml of THF is added dropwise under argon. The mixture is left stirring atordinary temperature for 20 hours. The THF is then evaporated, theresidue is taken up in ether, washing is carried out with water,separation is carried out by settling, drying is carried out andevaporation is carried out. 16 g of oily liquid are then vacuumdistilled, making it possible to obtain 11 g of monomer (III) (Yield:75% with respect to the dichlorinated derivative).

NMR spectra of the product obtained are shown in FIG. 1.

B) Conditions suitable for the electrochemical polymerization of themonomers (III)

Two methods of polymerization are described below:

1. Constant-potential electrolysis on a platinum sheet

1.2 g of monomer (III) (m=3) is dissolved in 25 ml of methylene chloridecontaining 0.1M Bu₄ NBF₄. This solution constitutes the anolyte arrangedin a compartment of a U-cell containing a second compartment (volume 20ml). The two compartments are separated by a sintered component with aporosity of 3. The anode consists of a platinum sheet with a totalsurface area of 20 cm². The cathode is made of graphite (immersedsurface area 15 cm²). A controlled potential of +1.0 V is applied to theanode. A current of 150 mA passes for 3 h 30 and 180 mg of a shiny blackconductive deposit are obtained.

The formation of a conductive polymer from III (m=2) can be demonstratedby cyclic voltametry (FIG. 3) under the following conditions:concentration 5.10⁻³ mol·l⁻¹. Electrolyte: acetonitrile containing 0.1mol·l⁻¹ Bu₄ NBF₄. Polished platinum microanode (surface area: 0.87 mm²).Sweep rate: 0.5 V·s⁻¹. Potentials shown with reference to the 0.1 MAg/AgNO₃ systems. Development of the deposit during the first 27 sweeps(the steady progression of the film, whose redox response is at +0.5 V,will be noted).

2. Constant-current electrolysis on graphite beads or grains

A solution of 1.7·10⁻² mol/l of (III) (m=3) in acetonitrile containing0.1M Bu₄ NBF₄ is placed in an electrolysis cell of Priam type 1.Circulation of the electrolyte is kept constant during thepolymerization operation at a flow rate of 2 l/min.

An electrolysis cell 1 of this type has been shown schematically in FIG.2.

A cathode 2 is composed either of one or a number of stainless steelgrid(s) 3 (apparent surface area: 1 dm²) or alternatively of a carbonpaper (for example the paper product from Carbon Lorraine). The oil coke4 (total mass: 50 g) is arranged at the centre of the case to form avoluminous anode between two porous walls 5 acting as spacer. The anodiccurrent supply is a titanium grid 6 covered with irridium oxide.

The electrolyte 7 enters at the upper part of the retaining vessel 8 anddeparts at the lower part 9 of the other side.

The current imposed is 0.6 A for 6 hours. In this case, the polymerdeposit obtained is 3.5 g. It is characterized by PER, IR and SEM (FIGS.4 and 5).

The values given above are given by way of example and are in no waylimiting.

With respect to a polymerization by constant-potentional electrolysis, apolymerization by constant-current electrolysis is representative of apreparation in an industrial environment. Moreover, as will be seensubsequently, the electrolysis technique used makes it possible toprepare directly elements which will then be assembled in order to formunits for the recovery of metal cations by encapsulation with polymerresins.

3. Reduction and dedoping of the conductive deposits

As was explained in prior documents (3,4), it is necessary to reduce thepolymer in order to remove part of the anions arising from theelectrolyte. This reduction can be carried out chemically orelectrochemically. It has been shown that the dedoping of the polymerwas more complete and simpler by the chemical route than by theelectrochemical route. Moreover, this technique can be applied to allthe polymers synthesized.

Dedoping of the deposits on a metal plate or on carbon can be easilyobtained [lacuna] treatment after the end of the electrolysis. Washingwith acetonitrile and drying in a vacuum oven at a temperature <100° C.The treatment can be carried out with commercial tetrabutyl- ortetraethyl ammonium hydroxides (source Fluka or Aldrich) at aconcentration of 10⁻¹ or 10⁻² M in acetonitrile. The reduction can alsobe carried out with tertiary amines, ammonium superoxides (prepared insitu by electrolysis of oxygen) or alkali metal superoxides (potassiumsuperoxide).

Neveretheless, it seems necessary for any application relating to anextraction of metal cations to subject to a treatment withtetraalkylammonium hydroxide, which makes the exchange between ammoniumions and metal ions easier.

The polymers formed from pyrrole ethers of type III (using the abilityof pyrrole to polymerize at the 2,2'-positions) have mesh structurespossessing a (probably large) number of polyether sites. ##STR15##

The anodic method thus makes it possible easily to form sites containing2, 4, 6, 8 or 10 oxygen atoms. Moreover, mixed electrolyses of twomonomers can easily modulate the size of the sites. Example: ##STR16##

Moreover, the formation of macrosites from no longer 2 but 3, indeed 4,monomers is not ruled out and can contribute to applications in theextraction of complex cations. Example for (III), m=2: ##STR17##

The possibility of obtaining not only these resins in the doped form butin the neutral form is also claimed.

These reduced polymers in the neutral form can behave as ion-exchangeresins and particularly have the property of extracting metal ionspresent in aqueous or organic solutions, provided that the resins aretreated beforehand with ammonium salts (operation as ion-exchangeresin). The affinity of the resins for alkali metal ions, alkaline-earthmetal ions, the silver ion and precious metal ions is claimed.

C) Industrial production and use of the polymeric resins synthesized

The method of industrial synthesis used may be that described inparagraph 2, i.e. a constant-current electrolysis on graphite beads. Infact, the use of voluminous anodes within which circulates theelectrolyte charged with monomer units will make it possible, during theelectrolysis, to obtain porous elements highly charged in"encapsulating" polymeric resins. At the end of electrolysis, dedopingof the polymer units is carried out in the same reaction loop.

If the electrolysis reactor used is a reactor containing voluminouselectrodes, it is sufficient, after dedoping the polymer, to remove thevoluminous anodes from the electrolysis vessel, to remove the currentsupply and then to enclose the top of the assembly in polypropylenewhich contains carbon granules coated with polymer resin. It thensuffices to combine a number of voluminous anodes in an appropriatereceptacle in order to form a porous stack capable of encapsulatingmetal cations present in low concentrations in industrial effluents.

The depletion in metal ions will be carried out by a simple percolationin this stack.

The present invention also relates to a process for the manufacture of afilter cartridge for the purification and/or recovery of metal ionscontained in a solution, characterized in that polymer units areproduced, by constant-current electrolysis, from an electrolyte chargedwith monomer units of the type described above and particularlyconsisting of a polyether carrying pyrrole or thiophene units bonded tothe ends of the chains of this polyether via, for each of theseheterocyclic compounds, either their carbon atoms in the 3,3'-positionsof their heterocycles or, as regards more particularly the pyrrolegroups, their respective nitrogen atoms, the 2- and 2'-positions ofthese heterocycles being, however, free of all substitutions or, at thevery most, substituted by easily removable groups, on a carbon-basedconductive support material contained in a reservoir possessing porouswalls belonging to a voluminous anode, at the end of electrolysis, thepolymer units thus obtained are dedoped in the same reaction loop, afterdedoping the polymer units, the means for supplying the current of thereservoir are removed, and then the said reservoir, which contains thesupport material coated with polymer units, is sealed off in order tothus obtain the said filter cartridge.

Finally, the present invention relates to a filter for the purificationand/or recovery of metal ions, more particularly of heavy metals such assilver, contained in solutions brought into contact with it,characterized in that it comprises at least one filter cartridgecomprising a reservoir possessing porous walls containing a conductivematerial, particularly carbon-based, coated with polymer resin of thetype described above and especially with polymer resin characterized bypolymeric chains containing a plurality of pyrrole or thiophene rings,if appropriate both at the same time, and interconnected to each othervia 2,2'-bonds, on the one hand, and polyether chains corresponding inparticular to the formulae --CH₂ --(--CH₂ --O--CH₂ --)_(m) --CH₂ -- withm equal to 3, 4 or 5, these chains being themselves bonded, via theirown ends, either to the carbon atoms in the 3,3'- positions respectivelyof these pyrrole or thiophene rings or, in the case of the pyrrolerings, to their nitrogen atoms.

FIG. 6 schematically shows an embodiment of such a filter 10 accordingto the invention comprising three identical filter cartridges 12, forexample of parallelepiped shape.

Each cartridge comprises a side bracing 14 made of rigid material forstructural maintenance of the cartridge and two rectangular walls 16made of porous material, for example of polypropylene, and thecartridges contain a carbon-based conductive material 18 coated withpolymer resin of the type according to the invention.

The filter comprises a parallelepiped casing 20 (chain-dotted line inthe figure), for example made of stainless steel, containing the threecartridges.

It is equipped with a removable upper plate 22, which makes it possibleto remove and replace the filter cartridges, equipped, in the upperpart, with a grasping handle 24.

Moreover, the filter comprises a section, for example tronconic 26, forentry of the liquid to be decontaminated and an identical section 28 fordeparture of the liquid after decontamination.

Such a filter makes it possible, for example, to obtain continuousdecontamination of effluent charged with radioactive Ag110 with a flowrate of the order of m³ /h.

REFERENCES

(1) J. Simonet and J. Rault-Berthelot, Prog. Solid St. Chem., (1991),21, 1-48 (and references cited).

(2) V. Le Berre, L. Angely, N. Simonet-Gueguen and J. Simonet, J.Electroanal. Chem., (1986), 206, 115.

(3) J. Simonet and C. Saboureau, French Patent Application (1990).

(4) Y. Gache and J. Simonet, J. Chim. Phys., (1992), 89, 1027.

I claim:
 1. A monomer consisting of an aliphatic polyether chaincontaining at least three ether units and pyrrole heterocyclic unitsbonded to ends of the chain via, for each of said heterocyclic unitseither carbon atoms in the 3,3'-positions of the heterocyclic units orthe respective nitrogen atoms, the 2-and 2'-positions of saidheterocyclic units being free of all substituents or substituted byeasily removable protective groups.
 2. A monomer according to claim 1,wherein the polyether chain forming part of said monomer is linear andcomprises ether units of the formula: --{(CH₂)_(p) --O}_(n) -- with pbeing an integer of 2 or 3 and n being an integer of 3 to 5, said chainbeing connected to α- and ω-positions to the pyrrole heterocyclic units.3. A monomer according to claim 1, wherein the chain has the formula:

    --(CH.sub.2).sub.q' --{(CH.sub.2).sub.p' --O--(CH.sub.2).sub.p" }.sub.m --(CH.sub.2).sub.q" --

with q' and q" being, independently of one another, equal to 1 or 2, pand p" being, independently of one another, equal to 1 or 2; and m isequal to 3, 4 or
 5. 4. A monomer according to claim 1, wherein themonomer is represented by the following formula: ##STR18## wherein m is3, 4 or
 5. 5. A monomer according to claim 1, wherein the monomer hasthe following structure: ##STR19##
 6. A monomer according to claim 1,wherein said monomer is represented by the following formula: ##STR20##wherein X is NH group, m is an integer of from 3 to 5 and q' and q" areindependently of one another equal to 1 or
 2. 7. A monomer according toclaim 5, wherein said polyether aliphatic chain is represented by theformula:

    --CH.sub.2 --(--CH.sub.2 --O--CH.sub.2 --).sub.m --CH.sub.2 --

wherein m is 3, 4 or
 5. 8. A monomer according to any one of claims 1 to7, wherein the monomer contains an acid group comprising a carboxy orsulpho group in the 3,3'-positions or 4,4'-positions of the heterocyclicunits.
 9. A monomer according to claim 8, wherein said monomer is in theform of an alkali metal salt of the pyrrole heterocyclic unit at the endof the polyether chain.
 10. A monomer according to claim 8, wherein themonomer is in the form of a potassium salt of the pyrrole heterocyclicunit.